Similar to eukaryotes, bacterial cells also use chemical signaling to communicate. Quorum sensing is a special type of cell-cell communication where some bacteria coordinate their behavior in response to changes in their population density. Such bacteria release small signaling molecules called auto-inducers. As the number of bacteria grows, the number of autoinducers in the surrounding environment increases. Once the concentration reaches a certain amount, known as the threshold concentration, the bacteria can detect the autoinducers. This pathway leads to the activation of intracellular second messengers, such as cyclic di-GMP, which regulate gene expression for various processes, including virulence and antibiotic resistance. Quorum sensing enables some bacterial species to act as a single multicellular organism. For instance, quorum sensing regulates bioluminescence in some bacteria and allows them to produce more light as a community than they could on their own. In addition, quorum sensing plays a key role in regulating the formation of bacterial biofilm, complex communities of bacteria that attach to surfaces. For example, dental plaque is a sticky bacterial biofilm that forms on teeth.